Electrical discharge machining

ABSTRACT

A method for electrical discharge machining a workpiece includes the steps of: presenting an elongate electrode to the workpiece with a spark gap therebetween; flowing a dielectric fluid in the gap; eroding the workpiece by electrical discharge between the tip of the electrode and the workpiece; displacing the electrode in a direction aligned with the long axis of the electrode to maintain the gap as the electrode wears and the workpiece is eroded; and simultaneously with the displacement, producing vibratory movement of the electrode, the vibratory movement being aligned with the long axis of the electrode.

This is a Divisional of U.S. application Ser. No. 13/991,231 filed Jun.3, 2013, which is a National Phase of International Application No.PCT/EP2011/066715 filed Sep. 27, 2011, which claims the benefit ofBritish Application No. 1020401.4 filed Dec. 2, 2010. The disclosures ofthe prior applications are hereby incorporated by reference herein intheir entireties.

FIELD OF THE INVENTION

The present invention relates to electrical discharge machining (EDM)and more particularly, but not exclusively, to so-called high speedelectrical discharge machining (HSEDM) utilised for forming holes incomponents such as blades for gas turbine engines.

BACKGROUND OF THE INVENTION

EDM is utilised with regard to processing of workpieces by sparkerosion. The workpiece and the electrode (usually made from graphite,copper or brass) are generally presented with a dialectric fluid betweenthem and are connected to a DC power supply (EDM generator) deliveringperiodic pulses of electric energy, such that sparks erode the workpieceby melting and vaporisation and so create a cavity or hole or otherwiseshape a workpiece. In order to provide for spark erosion, the workpieceand the electrode must have no physical contact and a gap is maintainedtypically through appropriate sensors and servo motor control. Erosiondebris must be removed from the erosion site and this usuallynecessitates a retraction cycle during conventional electrical dischargemachining. It is possible to utilise multiple electrodes in a singletool holder to allow several erosion and machining processes to beperformed at the same time and normally side by side.

In HSEDM a high pressure (e.g. 70 to 100 bar) dielectric fluid pump isutilised in order to supply dielectric fluid to the gap between theworkpiece and the electrode. As a result of the high pressurepresentation of the dielectric fluid, the process is more efficient thanconventional EDM, allowing more rapid removal of debris such thaterosion rates are far greater. With HSEDM there is no need forretraction cycles between stages of erosion for evacuation of debris, asthe high pressure flow of dielectric fluid in the gap between theworkpiece and the electrode is more efficient for the removal of debrisproduced by the erosion process. Thus generally the electrode is simplyfed forwards at a speed necessary to achieve the desired rate ofmaterial erosion and removal in accordance with the machining process.Continuous operation results in a significantly-faster machiningprocess.

In the attached drawings, FIG. 1 schematically illustrates a typicalHSEDM arrangement for the drilling of holes. The arrangement 1 comprisesan electrode holder 2 which presents an elongate electrode 3 to aworkpiece 4. Electrical discharge from the tip of the electrode isprovided through a direct current electrical power generator 5 such thata cavity or hole is drilled, formed or machined into the workpiece.Dielectric fluid is supplied at a relatively high pressure (70 to 100bar) to the cavity or hole defined progressively by a spark gap betweenthe electrode and the workpiece. This high pressure dielectric flow isachieved through a pump 6 which acts on a dielectric fluid supply 7 toforce the fluid under pressure as indicated into the gap between theelectrode and the workpiece. The high pressure flushes and removesdebris caused by the discharge process. A servo motor 8 or other deviceforces continuous movement of the electrode in the length direction ofthe electrode, driving the electrode into the workpiece. By monitoringthe gap voltage, the servo motor can maintain a gap of constant size.Due to the high pressure dielectric fluid flow, there is rapid removalof debris and therefore generally it is not necessary to have aretraction cycle of the electrode in order to allow flushing as withconventional EDM. Thus, in the normal course of events, the servo motorsimply moves the electrode down at the speed necessary to keep up with adesired rate of material removal and/or erosion. The constant motionproduced by the servo motor allows for rapid drilling, but if drillingis too rapid there is an increased likelihood of short circuiting. Insuch circumstances, the servo motor retracts the electrode to allowclearing of the electrical short circuits and debris, and thenreintroduces the electrode to reestablish the correct gap size forerosion.

HSEDM is used for drilling cooling holes and other features in turbineblades for gas turbine engines. Components such as turbine blades havevery strict requirements with regard to hole geometry and surfaceintegrity which can be met by HSEDM. However, HSEDM has high productioncosts and can lead to large variations in typical breakthrough time toform a hole. Also electrode wear necessitating re-working of componentscan be a problem. For example, it is not uncommon to have relativeelectrode wear factors which are greater than 100%, i.e. a greaterlength of electrode can be worn away than the depth of drilled hole.Electrode wear can also lead to tapering of the electrode, asillustrated in FIG. 2A and uneven wear in banks of electrodes, asillustrated in FIG. 2B. Electrodes that become tapered produce taperedholes, with a restriction at an exit end. Uneven electrodes in amultiple electrode tool result in some electrodes not fully penetratingthe workpiece to leave blocked holes. Alternatively, if the servo motorneeds to feed the electrodes deeper to complete the hole formation, theexcess electrode length in some of the electrodes can lead to backwallimpingement erosion and so damage other parts of the component. Suchbackwall impingement erosion is illustrated in FIG. 3, in which thedrilled through-hole 21 in turbine blade 22 continues in the drillingdirection 20 into a backwall as unplanned cavity 23. Thus skilledoperation of the HSEDM process can be essential.

WO 2009/071865 proposes an improved HSDEM process in which ultrasoniccavitation is induced within the pressurised dielectric fluid flow toenhance debris removal and thereby improve continuous machining.

SUMMARY OF THE INVENTION

However, there is a need for further improvements in electricaldischarge machining processes.

Accordingly, a first aspect of the present invention provides a methodfor electrical discharge machining a workpiece including the steps of:

-   -   presenting an elongate electrode to the workpiece with a spark        gap therebetween,    -   flowing a dielectric fluid in the gap,    -   eroding the workpiece by electrical discharge between the tip of        the electrode and the workpiece,    -   displacing the electrode in a direction aligned with the long        axis of the electrode to maintain the gap as the electrode wears        and the workpiece is eroded, and    -   simultaneously with the displacement, producing vibratory        movement of the electrode, the vibratory movement being aligned        with the long axis of the electrode.

Advantageously, the vibratory movement of the electrode can inducecorresponding vibrations in the dielectric fluid, which cause the fluidto form pulsating jets in the gap. These pulsating jets can help toclear debris from the spark gap, allowing machining to progress atgreater speeds. Also, the vibratory movement of the electrode can helpto reduce the occurrence of short-circuits between the electrode and theworkpiece which can lead to electrode retraction and workpiece damage.

The method may have any one or, to the extent that they are compatible,any combination of the following optional features.

Usually, the electrode has an axial bore, i.e. the electrode can betubular. The dielectric fluid can then be supplied in one directionthrough the bore and then flow over the outer surface of the electrode.For example, the fluid can exit the bore at an end of the electrode, andthen return in the opposite direction over the outer surface of theelectrode.

The electrode can be rotated about its long axis to reduce unevenelectrode wear and to improve hole circularity.

The dielectric fluid may be supplied to the gap at a pressure of from 70to 100 bar. High fluid pressures help to flush debris from the sparkgap.

The dielectric fluid may be supplied to the gap at an electricalresistivity of from 2 to 17 MΩ.cm. The dielectric fluid can be deionisedwater.

The vibratory movement may have a frequency of up to 500 Hz, andpreferably of up to 250 or 200 Hz. The vibratory movement may have afrequency of more than 50 Hz, and preferably of more than 80 Hz. Thevibratory movement may have a frequency of about 100 Hz.

Preferably, the vibratory movement is sinusoidal.

Preferably, the electrode is displaced by a servo system (e.g. based onone or more linear induction motors, or one or more linear actuatorssuch as piezo-electric actuators or pneumatic linear actuators combinedwith, for example, a lead-screw rotary motor) having a frequencyresponse of at least 1 kHz and more preferably of at least 10, 50 or 100kHz. Such devices can provide a high vibratory movement frequency. Afurther advantage of displacing the electrode using such a servo systemis that its frequency response can be of a similar order of magnitude tothe electrical discharge spark frequency (typically around 1-100 kHz)used in electrical discharge machining. Thus, the machining process canbe made more responsive to fast changes in spark gap conditions, leadingto a more stable and faster process. In contrast, many conventionalelectrical discharge machining systems are based on lead-screwservomotors which typically have maximum frequency responses of onlyabout 30 Hz and are thus less capable of maintaining a constant sparkgap.

The vibratory movement may have an amplitude of up to 200 microns, andpreferably of up to 75 microns. The vibratory movement may have anamplitude of more than 20 microns. The vibratory movement may have anamplitude of about 50 microns.

In order that the vibratory movement and fluid jets can clear the debrisefficiently from the spark gap, it is typically advantageous to erodethe workpiece while maintaining a larger spark gap size than is usualduring conventional electrical discharge machining. For example, for adisplacement velocity of the electrode of less than 1.5 mm/sec, thespark gap voltage (which is typically used as a measure of the spark gapsize) may be greater than 35V.

The step of flowing the dielectric fluid in the gap can include sendingpulsating jets of the fluid to the gap. Such pulsating jets can furtherimprove the rate of debris removal from the spark gap. Conveniently, thepulsating jets can have a pulse frequency which is the same as thefrequency of the vibratory movement of the electrode. In the case of anelectrode with an axial bore, the pulsating jets can be sent along thebore to the spark gap.

Any one or more of the rate of electrode displacement, the vibrationamplitude, and the vibration frequency can be varied as the workpiece iseroded, e.g. as conditions at the spark gap change.

According to the method, a single electrode may be presented to theworkpiece. Alternatively, a plurality of electrodes may besimultaneously presented to the workpiece.

A second aspect of the present invention provides an electricaldischarge machining apparatus including:

-   -   an elongate electrode,    -   a drive mechanism which displaces the electrode relative to, in        use, a workpiece, the displacement being in a direction aligned        with the long axis of the electrode, and maintaining a spark gap        between the electrode and the workpiece as the electrode wears        and the workpiece is eroded by the electrode,    -   a dielectric source which produces a dielectric fluid flow in        the gap, and    -   a vibration source which produces, simultaneously with the        displacement, vibratory movement of the electrode, the vibratory        movement being aligned with the long axis of the electrode.

Thus the apparatus is suitable for performing the method of the firstaspect. Accordingly, the apparatus may have any one or, to the extentthat they are compatible, any combination of the optional featurescorresponding to the optional features of the method of the firstaspect. For example, the apparatus may have any one or, to the extentthat they are compatible, any combination of the following optionalfeatures.

The electrode may have an axial bore, i.e. the electrode can be tubular.

The electrode can be rotated about its long axis.

The dielectric source may supply the dielectric fluid to the gap at apressure of from 70 to 100 bar.

The vibration source can produce vibratory movement having a frequencyof up to 500 Hz, and preferably of up to 250 or 200 Hz. The vibrationsource can produce vibratory movement having a frequency of more than 50Hz, and preferably of more than 80 Hz. The vibration source can producevibratory movement having a frequency of about 100 Hz.

Preferably, the drive mechanism and/or the vibration source has afrequency response of at least 1 kHz and more preferably of at least 10,50 or 100 kHz.

The vibration source can produce vibratory movement having an amplitudeof up to 200 microns, and preferably of up to 75 microns. The vibrationsource can produce vibratory movement having an amplitude of more than20 microns. The vibration source can produce vibratory movement havingan amplitude of about 50 microns.

Preferably, the vibration source produces a vibratory movement which issinusoidal.

Conveniently, the apparatus can include one or more linear inductionmotors which provides both the drive mechanism and the vibration source,the linear induction motor being coupled to the electrode to displacethe electrode relative to the workpiece, and to produce, simultaneouslywith the displacement, vibratory movement of the electrode.Advantageously, a linear induction motor can combine a high frequencyresponse with high positional accuracy.

However, alternatively, the apparatus can include one or more linearactuators which provide the vibration source, the linear actuators beingcoupled to the electrode to produce the vibratory movement of theelectrode. For example, the linear actuators can be piezo-electricactuators or pneumatic linear actuators. One option for such anarrangement is to operationally connect the one or more linear actuatorsto a reservoir for the dielectric fluid, such that, on activation of theactuators, pulsating jets of the fluid are sent from the reservoir tothe spark gap simultaneously with the production of vibratory movementof the electrode. Conveniently, one or more linear actuators can beretrofitted to an existing electrical discharge machining apparatus toconvert the apparatus into one according to the second aspect of theinvention. The drive mechanism can include a lead-screw servomotor whichis coupled to the electrode to displace the electrode relative to theworkpiece. The one or more linear actuators may share with theservomotor the drive mechanism task of maintaining a spark gap betweenthe electrode and the workpiece. For example, the linear actuators maydisplace the electrode up to a stroke limit of the actuators, whereuponthe drive mechanism feeds the electrode to reset the actuators. Thisallows the apparatus to benefit from the high frequency response andhigh positional accuracy of a typical linear actuator. Thus, moregenerally, the one or more linear actuators may combine with a separateservomotor to provide the drive mechanism. Alternatively, the one ormore linear actuators may be separate from the drive mechanism.

The apparatus may further include a tool holder which presents a singleelectrode. Alternatively, the apparatus may further include a toolholder which presents a plurality of electrodes to the workpiece.

In embodiments in which the dielectric source includes a reservoir forthe dielectric fluid, the vibration source, on activation, can vibrate apiston that generates corresponding pressure pulses in the dielectricfluid of the reservoir, the axial bore of the electrode opening to thereservoir such that the pressure pulses produce the fluid jets.Conveniently, the electrode can then be connected to the piston suchthat the piston and electrode vibrate in unison. For example, when oneor more linear actuators provide the vibration source, these can beconnected to the piston by corresponding flexure joints. The connectionto the piston can be direct, or indirect e.g. via a pressure cap andstopper arrangement. The electrode may enter the reservoir through anaperture in the cartridge, and preferably in the piston. The aperturemay have a seal formation which grips the electrode and prevents leakageof dielectric fluid from the reservoir at the aperture. The sealformation may be configured such that its grip on the electrode isactivated by the pressure of the dielectric fluid in the reservoir. Forexample, the seal formation may comprise a resilient body which iscompressed (e.g. by the piston) into sealing engagement with theelectrode under the action of the pressure of the dielectric fluid. Whenthe apparatus includes a tool holder which presents a plurality ofelectrodes to the workpiece, the tool holder may include the piston (andthe optional seal formation), which can then have a plurality ofrespective apertures for the electrodes. Indeed, more generally, thetool holder can take the form of a cartridge which also contains thereservoir. The piston is typically located at the lower end of thecartridge.

The apparatus may further include a computer-based control system forcontrolling the drive mechanism and the vibration source. The controlsystem can be adapted such that any one or more of: the rate of theelectrode displacement, the vibration amplitude, and the vibrationfrequency varies with electrode position. Typically, the apparatusfurther includes an electrical power supply which provides electricalpower to the electrode, and a sensor which measures the spark gap. Thecontrol system can then also control the sparking frequency and thespark gap, and further can be adapted so that either of both of theseparameters also varies with electrode position.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 schematically illustrates a typical HSEDM arrangement;

FIG. 2A shows at top an unworn electrode and at bottom a worn, taperedelectrode, and FIG. 2B shows a bank of differentially worn electrodes;

FIG. 3 shows a section through a turbine blade with undesirable backwallerosion;

FIGS. 4A, 4B and 4C show schematically stages of the electricaldischarge machining process with regard to erosion;

FIGS. 5A and 5B show schematically respectively front and side views ofan HSEDM apparatus;

FIG. 6 shows schematically a spark gap control system for the apparatusof FIGS. 5A and 5B;

FIGS. 7A, 7B, 7C and 7D show respective schematic cross-sections ofworkpieces and tubular electrodes during HSEDM drilling, thecross-sections in FIGS. 7A and 7C being without vibratory movement beingapplied to the electrodes, and the cross-sections in FIGS. 7B and 7Dbeing with vibratory movement being applied to the electrodes;

FIGS. 8A and 8B show respective Design of Experiment interaction plotsfor drilling speed plotted against Servomotor Speed and Gap Voltage, inboth cases with or without vibrations;

FIG. 9 shows typical plots of drilling depth against time obtained withand without vibratory movement being applied to an electrode;

FIGS. 10A and 10B show respective Design of Experiment interaction plotsfor cycle time plotted against peak current, and duty cycle , in bothcases with or without vibrations;

FIGS. 11A and 11B show schematically respectively front and side viewsof another HSEDM apparatus;

FIGS. 12A and 12B shows schematically front views of respectively thetool holder and the vibration plate of the apparatus of FIGS. 11A and11B; and

FIG. 13 shows schematically a close-up front view of the lower end ofthe electrode cartridge and the pressure cap of the apparatus of FIGS.11A and 11B.

DETAILED DESCRIPTION

Removal of debris during HSEDM is important in order to achieveappropriate machining speeds and consistency. Debris is removed by thedielectric flushing out debris in the time between the sparks. Thisprocess is shown schematically in FIGS. 4A-4C. A gas bubble, illustratedin FIG. 4A is generated by high temperatures as a result of sparkdischarge. This gas bubble then implodes as illustrated in FIG. 4B. Thetime between sparks, known as the “off time”, should be sufficientlylong to allow dielectric fluid flushing to remove the debris. The offtime determines the overall drilling cycle time for electric dischargemachining. Lack of adequate debris removal therefore results inincreased cycle times. Furthermore, poor debris removal increaseselectrode wear in the form of tapering. In FIG. 4A, as can be seen, anelectrode 30 has a spark gap 31 to a workpiece surface 32. Duringelectrical discharge a spark-induced plasma channel 33 creates debris 34from the workpiece surface as well as releasing some electrode debris35. Due to the heat of the spark, a bubble 36 is created within the highpressure dielectric fluid 37.

As illustrated in FIG. 4B, during the off time the bubble 36 implodes,allowing the debris 34, 35 to enter into the dielectric fluid flow 37.During this off time, in addition to the debris, molten metal ispartially removed from a spark generated crater 38. Any molten metalthat is not removed solidifies and becomes what is known as a recastlayer. Such recast layers can have detrimental effects in terms ofsurface modifications of the material from which the workpiece isformed.

FIG. 4C illustrates the association between the workpiece 32 and theelectrode 30 just prior to further electrical discharge machining. Thedebris 34, 35 is held in suspension within the dielectric 37 and istherefore flushed away under the relatively high pressure provided byHSEDM. Progressively craters 38 are formed across the surface of theworkpiece in order to erode and drill as required.

However, interruptions caused by inadequate removal of debris andconsequent short circuiting can limit HSEDM effectiveness.

FIGS. 5A and 5B show schematically respectively front and side views ofan HSEDM apparatus. The tool holder 106 for the electrodes 108 has beenomitted from the front view (a) so that other components of theapparatus can be visualised.

A linear induction servomotor 101 is coupled to a head carriage 103 bymeans of a motor rod 102. The head carriage is in turn mounted to alinear rail 115 (although in other embodiments, more than one linearrail may be used, or different types of linear guides can be employed,including linear air bearings). When the linear servomotor is activated,linear motion is thereby imposed on the head carriage.

An electrical connector 118 and a pneumatic chuck 104 are provided onthe head carriage 103. The connector 118 is connected to an electricalpower supply (omitted in FIGS. 2A and 2B) and transmits power across amating connector 117 to a row of elongate tubular electrodes 108 mountedto a tool holder 106. The pneumatic chuck 104 holds the tool holder tothe head carriage under an electric signal command.

The tool holder 106 has an electrode cartridge 105. A noseguide assembly111 carrying a noseguide 110 is coupled to a static part 114 of theapparatus by means of a chuck 112. The electrodes 108 and high-pressuredielectric fluid are contained within the electrode cartridge. Theelectrodes pass under clamps 107, 109 and out through the noseguide. Theclamp 107 is mounted beneath the electrode cartridge and consists of abar, with a rubber pad, that is pneumatically applied to nip theelectrodes during the drilling cycle. The clamp 109 is mounted on thenoseguide assembly and consists of a bar, with rubber pad, that ispneumatically applied to nip the electrodes during the electrode reefedcycle.

Compressed air is supplied to clamps 107, 109 through respectiveconnectors 116, 113. High-pressure dielectric fluid is fed to theelectrode cartridge 105 and the noseguide 110 through respectiveconnectors 119 and 120. Thus connectors 116, 119 are on the headcarriage 103, while connectors 113, 120 are on the static part 114 ofthe apparatus. The tubular electrodes are bathed in dielectric fluid ina reservoir contained within the electrode cartridge 105 so that thedielectric can flow both through and outside the electrodes. Ahigh-pressure (e.g. 70-100 bars) pump (omitted in FIGS. 2A and 2B)supplies dielectric fluid (e.g. deionised water) to the reservoir withinthe electrode cartridge 105 and thence to the machining spark gapbetween the electrodes and workpiece (e.g. blade) being drilled.

The linear induction servomotor 101 is capable of producing accelerationof up to 50 g in a mass of up to 10 Kg and can provide positionalaccuracy as small as 1 micron. In contrast to conventional rotarymotors, linear induction motors convert electrical energy directly intolinear movement, producing a straight-line force along the length of themotor. The linear servomotor is thus able simultaneously to displace theelectrodes 108 in a direction aligned with their long axes to maintainthe spark gap as the electrode wears and the workpiece is eroded, and toproduce vibratory movement of the electrodes, the vibratory movementbeing aligned with the long axes of the electrodes.

A control system for the apparatus of FIGS. 5A and 5B is shownschematically in FIG. 6. The linear servomotor 101 is controlled by adrive 121, i.e. an electronic power amplifier that delivers the powerrequired to operate the motor in response to low-level control signalssupplied by a controller 122 which sets the motor motion parameters. Acomputer 123 is used to input the desired motion parameters includingcharacteristics of a vibration sin wave 124 in terms of period (P) andamplitude (A), a displacement speed 125 (on which the sinusoidalvibration is superimposed) and a servo reference voltage 126. The smalldiameter tubular electrodes 108 and the workpiece 127 are connected toan electrical power supply 128, i.e. the EDM generator, which deliversperiodic pulses of energy 129 to the spark gap 130.

As machining occurs (i.e. high frequency sparks remove material fromboth electrodes 108 and workpiece 127), the linear servomotor 101displaces the tool holder 106 to which the electrodes are mounted at thedisplacement speed 125 to keep constant the spark gap 130 betweenelectrodes and workpiece. A meter 131 continuously measures the mean gapvoltage, which is compared with the servo reference voltage 126 by anumerical control (NC) unit 132. The tool holder 106 is moved downwardif the mean gap voltage is higher than the reference voltage and upwardwhen the mean gap voltage is lower than the reference voltage. Thelinear servomotor has a frequency response in excess of 1000 Hz, i.e.due to the dynamic characteristics of the linear servomotor and itscontrol system, the servomotor can respond to changes in the spark gapwithin 0.001 sec.

Key process variables (such as frequency and amplitude of vibration,speed of displacement and EDM generator parameters) can be varied duringthe drilling process according to the depth of holes being drilled. Thisvariation may be controlled by a program executed by the computer 123,together with the NC unit 132. An alternative approach that can be usedto change key process variables during the drilling process is to usesensors to measure spark gap conditions in a closed-loop system e.g.combined with artificial intelligence techniques such as neural networkor fuzzy logics. Such an approach could facilitate dynamic optimisationof the process variables.

The linear servomotor 101 can induce vibrations in the electrodes of upto 200 Hz with peak to peak amplitudes of up to 100 microns, and aresolution smaller than 0.1 microns. These vibrations inducecorresponding vibrations in the dielectric fluid which can improveremoval of debris from the spark gap. Furthermore, the servomotorpositional accuracy of 1 micron facilitates accurate control of thespark. In addition, the high frequency vibration creates gaps betweenthe electrode surfaces and the walls of the drilled hole which minimisethe occurrence of arcing.

More specifically, cooling holes in turbine blades can have diameters assmall as 0.38 mm and length-to-diameter-ratios of up to 80:1. Thediameter of an electrode employed to drill 0.38 mm holes is usually 0.33mm. If there is a requirement to drill a hole with diameter of 0.38 mmand length of 30 mm, the distance from the tip of the electrode to thenoseguide will be 30 mm at hole breakthrough. Such a slender electrodecan tend to tilt and touch the sidewall of the hole during the drillingprocess, provoking short-circuits and process interruption. Anotherproblem associated with the drilling of deep holes with small diametersis the removal of debris from the spark gap. This can be difficult evenwhen high-pressure dielectric fluid (of up to 100 bars) is employed. Theaccumulation of debris can provoke arcing and increase cycle times.These problems become more critical when multi-electrode drillingoperations are carried out, as the apparatus has just one servomotor tocontrol a plurality of spark gaps.

FIGS. 7A-7D show respective schematic cross-sections of workpieces 205and tubular electrodes 202 during HSEDM drilling. The workpieces aredrilled using multi-electrode tools 203 and high-pressure (70 to 100bars) dielectric fluid 201 supplied to the bore of the electrodes fromthe electrode cartridge (omitted). High frequency sparks 207, in theorder of 100 kHz, promote material removal both from electrodes andespecially from the workpieces.

FIGS. 7A is an example of the process without vibratory movement beingapplied to the electrodes 202. The resultant debris 206 from the processtends to accumulate in the spark gap and in the lower end of the holesas the dielectric pressure is insufficient to flush the debris out inthe exiting flow 204. The accumulation of debris can result in arcing,which damages the workpiece and increases cycle times. However, whenaxially aligned vibrations 210 are applied to the electrodes, as shownin FIG. 7B, the oscillating electrodes and holes being drilled act likereciprocating pumps in which the electrodes are the pistons and holesare the cylinders. The vibratory movement of the electrodes at afrequency of up to 500 Hz and peak to peak amplitude of up to 100microns pumps the dielectric fluid 212 and debris out of the spark gapand the holes. Thus the pumping action improves flushing 211, and can beincreased further when the vibrations are combined with pulsating jets209 of dielectric fluid sent to the spark gap through the axial bore ofthe electrode, the jet pulsations along the bore of the electrode havingthe same frequency as the electrode vibratory movement. An HSEDMapparatus which produces such synchronised pulsating jets is describedbelow in relation to FIGS. 10A-12B.

FIG. 7C is another example of the process without vibratory movementbeing applied to the electrodes 202. The tubular electrodes 202 tend toform cores 208 of workpiece material that remain uncut in the centres ofthe holes being drilled. Such a core may tilt and touch 213 the internalwall of the electrode, provoking short-circuits. In addition, theslender electrodes can move sideways 214 and touch the sidewall of theholes being drilled, again provoking short-circuits. Such short-circuitscause servo retraction and consequently lead to longer machining timesor to process interruptions. Moreover, the short-circuits can damage theworkpiece. However, when axially aligned vibrations 210 are applied tothe electrodes, as shown in FIG. 7D, small gaps 215, 216 can be moreeasily maintained between the cores and electrode bore, and between theelectrode outer surface and the hole sidewall. These gaps result fromdamage caused by the vibrations to the roughness asperities on thesurfaces of the electrodes and the workpiece, the asperities being thechannels for electrical current flow between the electrodes and theworkpiece.

Thus the vibration of the electrodes improves flushing and reducesshort-circuits, and, as a result, the servomotor can move downwards atfaster speeds.

Drilling trials were carried out using a multi-electrode tool withcapacity to hold 18 tubular electrodes. The diameter of the electrodeswas 0.31 mm and these were used to cut (in a single pass) 18 holes witha length of 4 mm. A Design of Experiments fractional factorial approachwas used to perform the experiments and analyse the results. The factorsused in the design are shown in the table below. The factor “Vibration”refers to the vibration produced in the electrode. The lower level (−1)of vibrations means that tests were carried out without vibrations,whereas the higher level (+1) means that the tests were carried out withvibrations. “Servomotor Speed” refers to the velocity with which theservomotor advances to keep the spark gap constant. “Gap Voltage” refersto the reference voltage, which is proportional to the spark gap size,i.e. a Gap Voltage at the higher level means that the size of the sparkgap is higher than at the lower level.

LEVEL FACTOR I II Vibration −1 +1 Servomotor Speed −1 +1 Gap Voltage −1+1

FIGS. 8A and 8B show interaction plots of the experimental parameters,i.e. drilling speed plotted again (a) Servomotor Speed and (b) GapVoltage for the different vibration levels. When vibrations are producedin the electrodes (dotted lines), smaller cycle times are achieved withthe servomotor speed at the lower level. In contrast, the higher servospeed decreased the cycle time when the vibrations were turned off. Asto gap voltage, when trials were carried out without vibrations,changing the value of the gap voltage did not affect cycle times. Incontrast, gap voltage had to be set at the higher level in order toreduce cycle times with electrode vibrations. When vibrations areapplied to the electrodes, a higher spark gap size helps the electrodeoscillations to remove debris from the spark gap.

FIG. 9 shows typical plots of drilling depth against time obtained withand without vibrations. Reductions in cycle times of nearly 50% can beachieved if vibrations are applied to the electrodes.

Further drilling trials were carried out to produce additionalinteraction plots. FIG. 10A shows plots of cycle time (i.e. time todrill a given hole depth) against peak current (+1=high peak current,−1=low peak current) for tests carried out with (+1) or without (−1)vibrations. HSEDM drilling assisted by vibrations is faster whencompared with drilling that is not assisted by vibrations. However, theimpact of vibrations is more significant when higher levels of peakcurrent are employed. FIG. 10B shows plots of cycle time against dutycycle (+1=high duty cycle, −1=low duty cycle) for tests carried out with(+1) or without (−1) vibrations, duty cycle being the ratio of thesparking time to the length of time required for one complete sparkingcycle (i.e. the time for sparking to take place and then for implosionof the gas bubble and removal of debris before the next sparking event).The impact of vibration becomes very significant for high levels of dutycycle, but is negligible at low levels of duty cycle.

The HSEDM apparatus described with reference to FIGS. 5A and 5B has alinear induction servomotor which both displaces the electrodes tomaintain the spark gap and produces the vibratory movement of theelectrodes. However, other configurations are possible, e.g. in whichthe displacement and vibration functions are driven by different partsof the apparatus. For example, FIGS. 11A and 11B show schematicallyrespectively front and side views of an HSEDM apparatus in which alead-screw servomotor drives the electrode displacement and separatepiezo-electric or pneumatic linear actuators drive the electrodevibration. The servomotor 301 has a coupling 302 to a lead-screw 303that turns the servo rotation into linear motion of a head carriage 304.

An electrical connector 321 and a pneumatic chuck 305 are provided onthe head carriage 304. The electrical connector is connected to anelectrical power supply (omitted in FIGS. 11A and 11B) and transmitspower across a mating connector 320 to tubular electrodes 311 mounted toa tool holder 308. The pneumatic chuck holds the tool holder to the headcarriage under an electric signal command.

The tool holder 308 has an electrode cartridge 307. A noseguide assembly314 carrying a static noseguide 313 is coupled to a static part 317 ofthe apparatus by means of a chuck 315. The electrodes 311 andhigh-pressure dielectric fluid are contained within the electrodecartridge. The electrodes pass under clamps 310, 312 and out through thenoseguide. The clamp 310 is mounted beneath the electrode cartridge andconsists of a bar, with rubber pad, that is pneumatically applied to nipthe electrodes during the drilling cycle. The clamp 312 is mounted onthe noseguide assembly and consists of a bar, with rubber pad, that ispneumatically applied to nip the electrodes during the electrode reefedcycle.

Compressed air is supplied to clamps 310, 312 through connectors 319,316. High-pressure dielectric fluid is fed to the electrode cartridge307 and the noseguide 313 through connectors omitted in FIGS. 11A and11B.

Two linear actuators 322 are assembled in a vibration plate 306 mountedto the tool holder 308 (in other embodiments only one linear actuator,or more than two linear actuators can be employed). FIGS. 12A and 12Bshow schematically front views of respectively the tool holder and thevibration plate. The vibration plate has flexure joints 323. Theelectrode cartridge 307 is attached to a static section 324 of thevibration plate, while a pressure cap 309 and the clamp 310 are attachedto a moving section 325 of the vibration plate. The pressure capcontains a rubber seal 326 and a plastic stopper 327. A piston 328 ismounted at the lower end of the electrode cartridge above the seal andthe stopper, with a seal ring 329 fluidly sealing the piston to thestatic section of the vibration plate. The piston, the seal and thestopper contain matching rows of holes through which the electrodes 311are passed.

FIG. 13 shows schematically a close-up front view of the lower end ofthe electrode cartridge 307 and the pressure cap 309. Just before thestart of the drilling process the electrodes 311 are clamped by theclamp 310 and dielectric fluid 330 at a pressure ranging from 70 to 100bars is supplied to a fluid reservoir defined within the electrodecartridge. The high-pressure fluid in the reservoir provokes a movementof the piston 328, which squeezes the seal 326 against the stopper 327.As a result, the holes in the seal reduce in size, gripping theelectrodes and sealing the electrode cartridge. A flow 331 of dielectricis supplied to the spark gaps through the bores of the electrodes andthrough flushing holes (omitted from FIGS. 11A to 13) in the noseguide.

The linear actuators 322 produce oscillations 332 in the moving section325 of the vibration plate 306, where the pressure cap 309 and clamp 310are mounted. The movement of the vibration plate induces vibrations(with frequencies of up to 500 Hz and peak-to peak amplitude up to 100microns) in the electrodes 311. Moreover, the oscillations of thepressure cap 309 induce pressure pulses in the dielectric fluid 330contained in the reservoir of the electrode cartridge 307. Thesepressure pulses produce high frequency pulsating jets 333 of dielectricfluid that are supplied to the spark gaps via the bores of theelectrodes. Advantageously, the combined effects of the pumping actionprovided by electrode oscillations and the high frequency pulses of thedielectric jets greatly improve the flushing of debris from the sparkgaps. Furthermore, the use of separate linear actuators to drive theelectrode vibration facilitates the retrofitting of such actuators ontoexisting HSEDM apparatuses.

A disadvantage of lead-screw servomotors is their typically lowfrequency response of about 30 Hz, which is not fast enough to respondto rapid changes to the spark gap. It is possible to increase thefrequency response of lead-screw servos by increasing the pitch and/orrotational speed. However, this affects the positional resolution of theelectrodes. Moreover, too high rotational speeds can cause the screw towhip or hit a resonant frequency causing uncontrolled vibrations andwild instability. However, by retrofitting a lead-screw servomotor withone or more linear actuators to drive electrode vibrations, the lowfrequency response can be side-stepped such that the retrofittedapparatus can be made to provide high frequency vibratory movement ofthe electrodes simultaneously with their displacement to maintain thespark gap. Also the dielectric fluid can be made to issue from the boresof the electrodes into the spark gaps as pulsed jets synchronised withthe electrode vibration to further enhance debris removal.

However, if screw resonance and whip can be avoided, by using e.g.appropriate software it is nonetheless possible to control a lead-screwservomotor to produce electrode vibrations superimposed on the linearmotion of the electrodes without the use of additional linear actuators.Although the response time of such an arrangement will be relativelylow, some benefits can be obtained, such as the ability to producepulsating jets of dielectric fluid and improved removal of debristhrough a dielectric fluid pumping action.

The apparatus of FIGS. 11A to 13 can be controlled by the control systemshown in FIG. 6.

In an operational variant, the task of keeping constant the size of thespark gap can be shared between the linear actuators 322 and thelead-screw servomotor 301. More specifically, the linear actuatorsprovide high positional precision and a high frequency response, butonly allow a maximum stroke about 200 microns. Thus, as well asvibrating the electrodes 311, the actuators can be used to displace theelectrodes to keep the spark gap constant up to the stroke limit of theactuators, whereupon the lead-screw servomotor re-feeds the electrodes.Indeed, a variant apparatus can have one or more linear actuators toprovide electrode vibration and displacement, and a linear inductionservomotor instead of a lead-screw servomotor to re-feed the electrodes.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. For example, an apparatus can have just one electrode.Another type of electrode tool holder can produce electrode rotationduring the drilling process. Accordingly, the exemplary embodiments ofthe invention set forth above are considered to be illustrative and notlimiting. Various changes to the described embodiments may be madewithout departing from the spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

1. A method for electrical discharge machining a workpiece including thesteps of: presenting an elongate electrode to the workpiece with a sparkgap therebetween, flowing a dielectric fluid to the gap, eroding theworkpiece by electrical discharge between the tip of the electrode andthe workpiece, using a servo system to displace the electrode in adirection aligned with the long axis of the electrode to maintain thegap as the electrode wears and the workpiece is eroded, andsimultaneously with the displacement, using the servo system to producevibratory movement of the electrode, the vibratory movement beingaligned with the long axis of the electrode; wherein the servo systemhas a frequency response of at least 1 kHz for displacing the electrodeto maintain the gap.
 2. A method according to claim 1, wherein theelectrode has an axial bore, and the dielectric fluid flows through thebore and into the gap.
 3. A method according to claim 2, whereinpulsating jets of the fluid are sent along the bore to the gap, thepulsating jets having a pulse frequency which is the same as thefrequency of the vibratory movement of the electrode.
 4. A methodaccording to claim 1, wherein the vibratory movement has a frequency ofup to 500 Hz.
 5. A method according to claim 1, wherein the vibratorymovement has a frequency of more than 50 Hz.
 6. A method according toclaim 1, wherein the dielectric source supplies the dielectric fluid tothe gap at a pressure of from 70 to 100 bar.
 7. A method according toclaim 1, wherein a plurality of the electrodes are simultaneouslypresented to the workpiece.
 8. An electrical discharge machiningapparatus including: an elongate electrode, a servo system whichdisplaces the electrode relative to, in use, a workpiece, thedisplacement being in a direction aligned with the long axis of theelectrode, and maintaining a spark gap between the electrode and theworkpiece as the electrode wears and the workpiece is eroded by theelectrode, a dielectric source which produces a dielectric fluid flow inthe gap, and the servo system being configured to produce,simultaneously with the displacement, vibratory movement of theelectrode, the vibratory movement being aligned with the long axis ofthe electrode; wherein the servo system has a frequency response of atleast 1 kHz for displacing the electrode to maintain the gap.
 9. Anapparatus according to claim 8, wherein the electrode has an axial bore,and the dielectric source flows the dielectric fluid into the gap alongthe bore.
 10. An apparatus according to claim 9, wherein the dielectricsource includes a reservoir for the dielectric fluid, and a vibrationsource is operationally connected to the reservoir, such that, onactivation of the vibration source, pulsating jets of the fluid are sentfrom the reservoir, along the bore and to the gap simultaneously withthe production of vibratory movement of the electrode.
 11. An apparatusaccording to claim 10, wherein the electrode enters the reservoirthrough an aperture having a seal formation which grips the electrode toprevent leakage of dielectric fluid from the reservoir at the aperture,the seal formation being configured such that its grip on the electrodeis activated by the pressure of the dielectric fluid in the reservoir.12. An apparatus according to claim 11, wherein the vibration source, onactivation, vibrates a piston that generates corresponding pressurepulses in the dielectric fluid of the reservoir, the axial bore of theelectrode opening to the reservoir such that the pressure pulses producethe fluid jets.
 13. An apparatus according to claim 12, wherein theelectrode is connected to the piston such that the piston and electrodevibrate in unison.
 14. An apparatus according to claim 8 wherein theservo system includes a linear induction motor.
 15. An apparatusaccording to claim 8 including one or more linear actuators whichprovide the vibration source and which combine with a separateservomotor to provide the drive mechanism, the linear actuators beingcoupled to the electrode to produce the vibratory movement of theelectrode.